Porous polytetrafluoroethylene (hereinafter PTFE), made by expansion by stretching at a temperature below the crystalline melt temperature of porous PTFE, has been known for some time. These porous, fibrillated materials and their manufacture were originally described by Gore in U.S. Pat. Nos. 3,953,566 and 4,187,390. They possessed the known attributes of PTFE while adding additional benefits resulting from their porous microstructure. They are typically hydrophobic, inert, strong and can be made to be thin and flexible. Applications for these materials include wire insulation, gaskets, waterproof and breathable rainwear and various surgically implantable medical devices such as vascular grafts, surgical repair patches, sutures and periodontal repair materials.
The greatest strength of these materials is typically in the direction in which they were expanded by stretching, the same direction in which the fibrils of the porous microstructure are oriented. Thus uniaxially expanded materials are predominantly strong in the single direction in which they were expanded.
Multiaxially expanded porous PTFE sheet materials are known, such as GORE-TEX.RTM. Soft Tissue Patch and GORE-TEX.RTM. Cardiovascular Patch (W. L. Gore & Associates, Inc., Flagstaff, Ariz.). These materials are implantable surgical repair patch materials which have fibrils oriented in virtually all directions within the plane of the sheet and substantially equal strength in all directions within the plane of the sheet. The strongest presently available porous PTFE sheet material is believed to be GORE-TEX.RTM. Cardiovascular Patch which has a mean tensile strength of about 29.8 megapascals, a mean matrix tensile strength of 133.6 megapascals, a mean secant modulus of 46.8 megapascals, and a mean matrix secant modulus of 209.8 megapascals, with all data taken from randomly selected directions within the plane of the material.
Other references which teach the manufacture of porous PTFE sheet materials include U.S. Pat. No. 4,482,516 to Bowman et al., which describes a biaxially stretched film, and U.S. Pat. Nos. 4,385,093 and 4,478,665 to Hubis which teach a method of making laminated, porous PTFE sheet materials having good bond strengths between the adjacent layers.